Epitaxial formation support structures and associated methods

ABSTRACT

Epitaxial formation support structures and associated methods of manufacturing epitaxial formation support structures and solid state lighting devices are disclosed herein. In several embodiments, a method of manufacturing an epitaxial formation support substrate can include forming an uncured support substrate that has a first side, a second side opposite the first side, and coefficient of thermal expansion substantially similar to N-type gallium nitride. The method can further include positioning the first side of the uncured support substrate on a first surface of a first reference plate and positioning a second surface of a second reference plate on the second side to form a stack. The first and second surfaces can include uniformly flat portions. The method can also include firing the stack to sinter the uncured support substrate. At least side of the support substrate can form a planar surface that is substantially uniformly flat.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/961,370 filed on Dec. 6, 2010, now U.S. Pat. No. 8,187,901, andclaims priority to U.S. Provisional application No. 61/267,134 filed onDec. 7, 2009, each of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is related to epitaxial formation structures andassociated methods of manufacturing epitaxial formation structures andsolid state lighting (“SSL”) devices.

BACKGROUND

SSL devices generally use semiconductor light emitting diodes (“LEDs”),organic light emitting diodes (“OLED”), and/or polymer light emittingdiodes (“PLED”) as sources of illumination rather than electricalfilaments, plasma, or gas. For example, FIG. 1 is a schematiccross-sectional diagram of a conventional indium-gallium nitride (InGaN)LED 10. As shown in FIG. 1, the LED 10 includes a substrate material 12(e.g., silicon), N-type gallium nitride (GaN) 14, GaN/InGaN multiplequantum wells (“MQWs”) 16, and P-type GaN 18. The LED 10 also includes afirst contact 20 on the P-type GaN 18 and a second contact 22 on theN-type GaN 14. During manufacturing, the N-type GaN 14, the GaN/InGaNMQWs 16, and the P-type GaN 18 are formed on the substrate material 12via metal organic chemical vapor deposition (“MOCVD”), molecular beamepitaxy (“MBE”), liquid phase epitaxy (“LPE”), hydride vapor phaseepitaxy (“HVPE”), and/or other epitaxial growth techniques, each ofwhich is typically performed at elevated temperatures.

One operational difficulty of forming the LED 10 is that the N-type GaN14, the GaN/InGaN MQWs 16, and the P-type GaN 18 may be delaminated fromthe substrate material 12 and/or otherwise damaged duringhigh-temperature epitaxial growth and/or cool-down thereafter.Typically, the substrate material 12 includes silicon (Si), sapphire(Al₂O₃), silicon carbide (SiC), and/or other “non-native” materialsbecause “native” materials (e.g., GaN or InGaN) with usable dimensionsare difficult to produce. The non-native substrate materials havedifferent coefficients of thermal expansion (“CTEs”) than the GaN/InGaNmaterials 14, 16, and 18. For example, the CTE of silicon issubstantially less than that of GaN, and the CTE of sapphire issubstantially greater than that of GaN. Such CTE differentials inducethermal stress as the wafer cools, which warp the substrate material 12and/or cause crystal defects in epitaxial GaN/InGaN materials 14, 16,and 18. Additionally, the non-native substrate materials that facilitateparticularly good epitaxial growth, such as Si(1,1,1) silicon wafer, canbe expensive. Accordingly, several improvements in reliably andcost-effectively manufacturing SSL devices may be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an LED device inaccordance with the prior art.

FIG. 2 is an exploded view of an apparatus for manufacturing epitaxialformation support substrates in accordance with embodiments of thetechnology.

FIGS. 3A-C are schematic cross-sectional views of a process formanufacturing epitaxial formation support substrates in accordance withembodiments of the technology.

FIGS. 4A-D are schematic cross-sectional views of a process formanufacturing an SSL device in accordance with embodiments of thetechnology.

FIG. 5 is an exploded view of an apparatus for manufacturing epitaxialformation support substrates in accordance with other embodiments of thetechnology.

FIG. 6 is an exploded view an apparatus for manufacturing epitaxialformation support substrates in accordance with further embodiments ofthe technology.

DETAILED DESCRIPTION

Various embodiments of apparatuses for manufacturing epitaxial formationsupport substrates and associated methods of manufacturing epitaxialformation structures and solid state lighting (“SSL”) devices aredescribed below. As used hereinafter, the term “SSL device” generallyrefers to devices with semiconductor light-emitting diodes (“LEDs”),polymer light-emitting diodes (“PLEDs”), organic light-emitting diodes(“OLEDs”), or other types of solid state devices that convert electricalenergy into electromagnetic radiation in a desired spectrum.Additionally, the term substrate refers to supports for individual SSLdevices and larger wafers that can support a plurality of SSL devices. Aperson skilled in the relevant art will also understand that thetechnology may have additional embodiments, and that the technology maybe practiced without several of the details of the embodiments describedbelow with reference to FIGS. 2-6.

FIG. 2 is an exploded view of an apparatus 200 for forming epitaxialformation support substrates in accordance with embodiments of thetechnology. The apparatus 200 can include a plurality of referenceplates 204 (identified individually as first, second, third, and fourthreference plates 204 a-d, respectively) sequentially stacked above oneanother. As shown in FIG. 2, a plurality of support substrates 202(identified individually as a first-third support substrates 202 a-c)can be positioned between the reference plates 204 to form a stack 206of alternating reference plates 204 and support substrates 202. In otherembodiments, more than one support substrate 202 can be positionedbetween two consecutively stacked reference plates 204 (e.g., the firstreference plate 204 a and the second reference plate 204 b).

The support substrates 202 can be made from a polycrystalline ceramicmaterial having a coefficient of thermal expansion (CTE) substantiallysimilar to the CTE of N-type gallium nitride (GaN) and/or otherIII-nitrides. For example, the support substrates 202 can be made fromcompositions of Si3N4, TiN, ZrN, HfN, AlN, SiO2, Al2O3, AlON, TiC, ZrC,HfC, SiC, Y2O3 and/or other suitable polycrystalline ceramics. In someembodiments, the support substrates 202 can include impurities that areinconsequential to the thermal characteristics of the support substrates202 as a whole. In other embodiments, the support substrates 202 caninclude sintering agents to aid in subsequent firing processes.Additionally, the support substrates 202 can include agents used toalter properties associated with the polycrystalline ceramic.

The support substrates 202 can be formed by tape casting a ceramicslurry into a putty-like material. The ceramic slurry can be pressed orextruded into a sheet having a desired thickness, and the supportsubstrates 202 can be cut from the sheet while in a green (i.e., unfiredstate). In other embodiments, the support substrates 202 can beindividually formed from the ceramic slurry into desired shapes andsizes. For example, the support substrates 202 can be shaped into discsas shown in FIG. 2 and/or other suitable shapes for supporting epitaxialgrowth of SSL structures. In further embodiments, the support substrates202 can be sized to compensate for shrinkage of the ceramic duringsubsequent firing. For example, if the ceramic shrinks 30% duringfiring, the support substrates 202 can have an initial sizeapproximately 50% larger than a desired size.

As shown in FIG. 2, each reference plate 204 can include a first surface208 a and a second surface 208 b opposite the first surface 208 a. Thefirst and second surfaces 208 can be uniformly flat and planar. Thereference plates 204, for example, can be polished, lapped, and/orotherwise machined to increase the uniformity and flatness of the firstand/or second surfaces 208. As such, the reference plates 204 are madefrom rigid materials that maintain the uniformity and flatness ofportions of first and second surfaces 208 a-b during firing. Forexample, the reference plates 204 can be made from aluminum nitride,sapphire, and/or other suitable materials that will maintain the desiredsurface finish during and after firing.

The planar first and second surfaces 208 can press the supportsubstrates 202 while the ceramic material is in a green state to form atleast one side of each support substrate 202 into a correspondinglyplanar surface. In the embodiment illustrated in FIG. 2, for example,each support substrate 202 has a first side 210 a and a second side 210b opposite the first side 210 a. Accordingly, the second surface 208 bof the first reference plate 204 a can contact the second side 210 a ofthe first support substrate 202 a, the first surface 208 a of the secondreference plate 204 b can contact the first side 210 b of the firstsupport substrate 202 a, and the second surface 208 b of the secondreference plate 204 b can contact a second side 210 b of thesubsequently stacked second support substrate 202 b. The third andfourth reference plates 204 c and 204 d can be similarly stacked on thesecond and third support substrates 202 b-c such that the first andsecond sides 210 of the support substrate 202 b-c contact correspondingfirst and second surfaces 208 of the third and fourth reference plates204 c-d. In other embodiments, the first and/or second surfaces 208 caninclude uniformly flat portions and/or the reference plates 204 caninclude only one uniformly flat portions. For example, the first surface208 a of the first reference plate 204 a need not be uniformly flatbecause the first surface 208 a does not contact any of the supportsubstrates 202.

The apparatus 200 can reduce post-firing machining of the fired supportsubstrates 202 because the reference plates 204 flatten the first side210 a or the second side 210 b of the support substrates 202 duringfiring. For example, the first and/or second sides 210 generally need atmost one machine process to form the desired flat surface on the firedsupport substrates 202. Each flattened side 210 provides a supportsurface for a formation structure (described below) on which SSLstructures can epitaxially grow. The flat surfaces of the supportsubstrates 202 can reduce or substantially eliminate warp and thetranslation of warp from the support substrate 202 to the SSL structureduring epitaxial growth. For example, warp from the support substrate202 to the SSL structure can be less than 25 micrometers.

FIGS. 3A-D are schematic cross-sectional views that illustrate a processfor manufacturing epitaxial formation support substrates in accordancewith embodiments of the technology. The process can use the apparatus200 described in FIG. 2 to form the support substrates 202 such thatthey facilitate uniform epitaxial growth. More specifically, FIG. 3Ashows the process can include stacking a plurality of uncured supportsubstrates 202 between the plurality of reference plates 204. Thereference plates 204 and the uncured support substrates 202 can bestacked in an alternating manner to form the stack 206. For example, asshown in FIG. 3A, the uncured support substrates 202 can be positionedin the stack 206 such that the first and second sides 210 of eachsupport substrate 202 contacts the corresponding first and secondsurfaces 208 of the reference plates 204. In other embodiments, thestack 206 can include more or less uncured support substrates 202positioned between reference plates 204 and/or multiple reference plates204 can be positioned over each uncured support substrate 202. Infurther embodiments, only one side 210 of each uncured support substrate202 can contact one planar surface 208.

As shown in FIG. 3B, the process can also include sintering the stack206 in a firing furnace 312. The firing furnace 312 can be a suitablefurnace for sintering ceramic materials, such as the uncured supportsubstrates 202. During this step, the heat from the firing furnace 312and the gravitational force from the reference plates 204 can flatten atleast one of the sides 210 of the uncured support substrates 202 into aplanar surface corresponding to the uniformly flat surfaces 208 of thereference plates 204. In other embodiments, external pressure and/oradditional weights can also be applied to the stack 206 during firing tofurther flatten the sides 210.

After the ceramic material of the support substrates 202 is cured and atleast one side 210 has at least a generally planar surface, each of thesupport substrates 202 can be separated from the stack 206. To easeremoval of the support substrates 202 from the stack 206, a releaseagent can be used to prevent the support substrates 202 from bonding orotherwise affixing to the reference plates 204. For example, in someembodiments, the release agent may be applied to the reference plates204 and/or to the support substrates 202 before to stacking Suitablerelease agents can include, for example, boron nitride.

FIG. 3C shows the first support substrate 202 a after removal from thestack 206. The first support substrate 202 a can have a planar surface314 at the first side 210 a corresponding to the uniformly flat firstsurface 208 a of the second reference plate 204 b. In some embodiments,the planar surface 314 can be flat enough to support uniform, epitaxialgrowth of SSL structures. In other embodiments, the planar surface 314can be machined to produce a more uniformly planar surface for moreuniform epitaxial growth of the SSL structures. As shown in theembodiment illustrated FIG. 3C, the planar surface 314 can be planarizedby removing a portion of the first support substrate 202 a to form amore planar surface 314′ (shown in broken lines) to support uniformepitaxial growth. However, subsequent machining of the planar surface314 is minimal, and, in some embodiments, can be limited to a singleprocess. Suitable machine processes can include polishing, lapping,chemical-mechanical planarization or other processes that can be ahighly planar surface 314 on the support substrates 202.

FIGS. 4A-D are schematic cross-sectional views of a process formanufacturing SSL devices on epitaxial formation support substrates inaccordance with embodiments of the technology. More specifically, theSSL devices can be formed on one of the support substrates 202manufactured using the process described with reference to FIGS. 3A-C.Even though only certain method steps are illustrated in FIGS. 4A-D, themethod for forming the SSL devices can also include other stages forforming a lens, a mirror, a carrier structure, conductive interconnects,electrical contacts, and/or other suitable mechanical/electricalcomponents (not shown).

As shown in FIG. 4A, the process can include attaching a formationstructure 418 to the planar surface 314 of the support substrate 202 toform a template structure 420. The formation structure 418 can be madefrom a material that facilitates epitaxial growth of III-nitridedevices. In some embodiments, for example, the formation structure 418can include silicon (Si), at least a portion of which has the Si(1,1,1)crystal orientation. In other embodiments, the formation structure 418can include silicon with other crystal orientations (e.g., Si(1,0,0)),aluminum gallium nitride (AlGaN), GaN, SiC, Al₂O₃, zinc oxide (ZnO₂),gallium arsenide (GaAs), a combination of the foregoing materials,and/or other suitable materials.

In the embodiment illustrated in FIG. 4A, the formation structure 418includes a bonding region 422 configured to bond with the planar surface314 of the support substrate 202. The bonding region 422 may be dopedwith an exfoliation agent (e.g., hydrogen (H or H₂), boron (B), helium(He), or any combination thereof) using ion implantation and/or othersuitable techniques. In other embodiments, the bonding region 422 may beomitted. In some embodiments, the formation structure 418 can beattached to the planar surface 314 using solid-solid bonding techniques.For example, the formation structure 418 and the support substrate 202may be mechanically pressed against each other while being heated to abonding temperature (e.g., 300° C.). In other embodiments, the formationstructure 418 and the support substrate 202 may be attached using anadhesive material (not shown) and/or other suitable techniques.

FIG. 4B shows another step in the process in which the formationstructure 418 is thinned such that thermal expansion characteristics ofthe template structure 420 are dominated by the CTE of the supportsubstrate 202. For example, the remaining formation structure 418 canhave a thickness of about 10 nanometers to about 2 micrometers, or canhave other suitable thicknesses such that the CTE temperature dependencyof the template structure 420 generally corresponds to that of thesupport substrate 202. In one embodiment, the portion of the formationstructure 418 can be removed through exfoliation of the dopant (e.g.,hydrogen (H or H₂), boron (B), or helium (He), or any combinationthereof) at a temperature of about 250° C. for hydrogen and boroncompounds, and about 400° C. for hydrogen or hydrogen and heliumcompounds. In other embodiments, the portion of the formation structure418 may be removed using CMP, ECMP, dry etching, wet etching, and/orother suitable material removal techniques.

As shown in FIG. 4C, the process can further include forming an SSLstructure 424 on the formation structure 418. In the illustratedembodiment, the SSL structure 424 can include a first semiconductormaterial 426, an active region 428, and a second semiconductor material430 formed via MOCVD, MBE, LPE, HVPE, and/or other suitable epitaxialgrowth techniques. In other embodiments, the SSL structure 424 can alsoinclude other suitable components, such as a buffer material thatfacilitates the formation of the first and second semiconductormaterials 426 and 430 and the active region 428 on the formationstructure 418. In further embodiments, the SSL structure 424 can includeadditional bonding and seed layers to facilitate bonding and/orepitaxial growth.

In certain embodiments, the first semiconductor material 426 can includeN-type GaN (e.g., doped with silicon (Si)), and the second semiconductormaterial 430 can include P-type GaN (e.g., doped with magnesium (Mg)).In other embodiments, the first semiconductor material 426 can includeP-type GaN, and the second semiconductor material 430 can include N-typeGaN. In further embodiments, the first and second semiconductormaterials 426 and 430 can individually include at least one of galliumarsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium arsenidephosphide (GaAsP), gallium(III) phosphide (GaP), zinc selenide (ZnSe),boron nitride (BN), AlGaN, and/or other suitable semiconductormaterials.

The active region 428 can include a single quantum well (“SQW”), MQWs,and/or a bulk semiconductor material. As used hereinafter, a “bulksemiconductor material” generally refers to a single grain semiconductormaterial (e.g., InGaN) with a thickness greater than about 10 nanometersand up to about 5 micrometers. In certain embodiments, the active region428 can include an InGaN SQW, GaN/InGaN MQWs, and/or an InGaN bulkmaterial. In other embodiments, the active region 428 can includealuminum gallium indium phosphide (AlGaInP), aluminum gallium indiumnitride (AlGaInN), and/or other suitable materials or configurations.

The template structure 420 can facilitate uniform epitaxial growth ofthe first and second semiconductor materials 426 and 430 and the activeregion 428. For example, the planar surface 314 can provide a uniformsupport that reduces warp from the support substrate 302 to the SSLstructure 424. Additionally, the template structure 420 can reducethermal stress during epitaxial growth of the SSL structure 424 becausethe CTE of the template structure 420 is dominated by the CTE of theceramic support substrate 202. Thus, the template structure 420 canepitaxially grow SSL structures 424 that have substantially uniformthicknesses and substantially uniform performance characteristics.

As shown in FIG. 4D, the process can also include removing the supportsubstrate 202 from the formation structure 418 and the SSL structure 424such that the SSL structure 424 can be used to as part of a device(e.g., cell phone, light, computer). In some embodiments, removing thesupport substrate 202 can include contacting the support substrate 202containing polycrystalline AlN with potassium hydroxide (KOH) at atemperature of about 100° C. It has been observed that KOH can reactwith polycrystalline AlN in the support substrate 202 until a diffusionbarrier containing Si₃N₄ is exposed. As a result, the diffusion barriercan functions as an etch stop. In other embodiments, the supportstructure 202 can be removed using CMP, ECMP, and/or other suitableremoval techniques.

FIG. 5 is an exploded view of an apparatus 500 for forming epitaxialformation support substrates in accordance with other embodiments of thetechnology. Several features of the apparatus 500 are generally similarto the features of the apparatus 200 of FIG. 2 and are accordingly notdescribed in detail below. As shown in FIG. 5, the reference plates 204can further include spacers 532 that can separate the reference plates204 apart from one another by a height H. The height H can be less thana thickness of the support substrates 202 to ensure the first and secondsurfaces 208 contact the corresponding first and second sides 210 of thesupport substrates 202. The spacers 532 can also facilitate asubstantially even pressure distribution on each of the supportsubstrates 202. For example, the spacers 532 can prevent the supportsubstrates 202 near the bottom of the stack 206 from receiving morepressure than the support substrates 202 near the top of the stack 206.Additionally, the height H of the spacers 532 can take into account theshrinkage of the material of the support substrates 202 during sinteringto ensure the sides 210 remain in contact with the planar surfaces 208.

As shown in the embodiment illustrated in FIG. 5, each reference plate204 can includes a plurality of the spacers 532 around peripheralportions of the first surfaces 208 a. In other embodiments, the spacers532 can be a single piece, such as a circular protrusion, that canencircle the support substrates 202. The spacers 532 can be affixed tothe reference plates 204 using glue and/or other suitable fasteningmethods. In other embodiments, the spacers 532 can be integrally formedwith the reference plates 204. The spacers 532 can be made from a rigid,semi-rigid, and/or flexible material that can withstand the temperaturesof firing. For example, in some embodiments, the spacers 532 can be madefrom aluminum oxide. Additionally, the spacers 532 can be made from amaterial that can compress to a desired height (e.g., a desired heightof the support substrate 202) when the reference plates 204 are stackedand/or pressure is applied to the stack 206. In other embodiments, thereference plates 204 can be spaced apart by a locking mechanism thatensures the reference plates 204 are not spaced an undesirable distanceaway from one another (e.g., too close, to far) such that asubstantially uniform pressure is applied to each of the supportsubstrates 202.

FIG. 6 is an exploded view an apparatus 600 for forming epitaxialformation support substrates in accordance with further embodiments ofthe technology. Several features of the apparatus 600 are generallysimilar to the features of the apparatus 200 of FIG. 2 and areaccordingly not described in detail below. As shown in FIG. 6, thereference plates 204 can increase in thickness from a first thickness T₁for the first reference plate 204 a to a thickness T₄ for the fourthreference plate 204 d. The mass of the reference plates 204 a-daccordingly increases as they are spaced higher in the stack 206. Thus,the support substrates 202 positioned closer to the top of the stack 206do not experience disproportionately less pressure than the supportsubstrates 202 near the bottom of the stack 206. In other embodiments,the reference plates 204 can be made from different materials such thatthe reference plates 204 have a greater mass as the reference plates 204are spaced closer to the top of the stack 206. In other embodiments, thereference plates 204 can compensate for different pressures within thestack 206 using another suitable method.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, the embodiments described above show a stack offour reference plates. However, other embodiments can include more orless reference plates. Many of the elements of one embodiment may becombined with other embodiments in addition to or in lieu of theelements of the other embodiments. Accordingly, the disclosure is notlimited except as by the appended claims.

I claim:
 1. A method of manufacturing an epitaxial formation supportsubstrate, comprising: positioning an uncured support substrate betweena first reference plate and a second reference plate to form a stack,the uncured support substrate having a side in contact with a formationportion of the first reference plate and coefficient of thermalexpansion (CTE) substantially similar to N-type gallium nitride (GaN);firing the stack to sinter the uncured support substrate; and removingthe support substrate from between the first and second referenceplates, wherein the side of the support substrate includes a planarsurface that is at least substantially uniformly flat.
 2. The method ofclaim 1 wherein the formation portion is substantially uniformly flat.3. The method of claim 1 wherein: the side of the uncured supportsubstrate is a first side; the uncured support substrate includes asecond side opposite the first side; and positioning the uncured supportsubstrate between the first and second reference plates furthercomprises contacting the second side with a formation portion of thesecond reference plate.
 4. The method of claim 1, further comprisingmachining the planar surface of the support substrate to increase theflatness of the planar surface, wherein machining is limited to onemachine process.
 5. The method of claim 1, further comprising attachinga formation structure to the planar surface of the support substrate,the formation structure including an Si(1,1,1) portion.
 6. The method ofclaim 1 wherein: the uncured support substrate is one of a plurality ofuncured support substrates; the first and second reference plates aretwo of a plurality of reference plates; and the method further comprisesalternatingly stacking the uncured support substrates and the referenceplates to form the stack.
 7. The method of claim 1, further comprisingpositioning at least one spacer between the first and second referenceplates, the spacer having a height less that of the uncured supportsubstrate.
 8. The method of claim 1, further comprising applying areleasing agent between the side of the uncured support substrate andthe first reference plate.
 9. The method of claim 1 wherein the uncuredsupport substrate is one of a plurality of uncured support substratesand wherein the method further comprises: forming a sheet ofpolycrystalline ceramic from a ceramic slurry; and forming the pluralityof uncured support substrates from the sheet.
 10. A method ofmanufacturing a solid state lighting (SSL) device, comprising: forming asupport substrate between reference plates, the support substrate havinga coefficient of thermal expansion (CTE) substantially similar to a CTEof N-type gallium nitride (GaN), and a planar surface that is at leastsubstantially uniformly flat; and attaching a formation structure to theplanar surface to form a template structure, the template structurehaving a CTE substantially similar to the CTE of the support substrate.11. The method of claim 10, further comprising epitaxially growing anSSL structure on the formation structure.
 12. The method of claim 11wherein epitaxially growing the SSL structure includes forming a firstsemiconductor material, an active region, and a second semiconductormaterial on the formation structure, the first semiconductor materialcontaining N-type gallium nitride (GaN), the second semiconductormaterial containing P-type GaN, and the active region including at leastone of a bulk indium gallium nitride (InGaN), an InGaN single quantumwell, and GaN/InGaN multiple quantum wells.
 13. The method of claim 11wherein epitaxially growing the SSL structure transmits less than 25micrometers of warp from the template structure to the SSL structure.14. The method of claim 10 wherein the formation structure has a CTEdifferent from the CTE of the support substrate.
 15. The method of claim10 wherein forming the support substrate between reference platescomprises: positioning an uncured polycrystalline ceramic supportsubstrate between first and second reference plates, the uncuredpolycrystalline ceramic support substrate having a first side facing aformation portion of the first reference plate and a second side facinga formation portion of the second reference plate; and firing theuncured polycrystalline ceramic support substrate between the first andsecond reference plates.
 16. The method of claim 10 wherein theformation portions of the first and second reference plates issubstantially uniformly flat.